GPS lag

Of course it's a couple of seconds behind. Finding a GPS position isn't instantaneous even with a continuous lock, and then it has to plot this on the screen and animate it. Google Maps is farther behind than the in-car GPS in my experience.

 

I have been using GPS for almost twenty years, starting with the Garmin II Plus (the map in it only had major highways!) Even when they started including turn-by-turn directions, the one thing that I learned early on is to use the GPS only for general guidance to let me know that I am in the ballpark, I always make the actual turns based on visual sighting of the street sign. When the GPS says the next turn is 0.10 miles away that's when I start looking for it. Even though I can't always read the street sign that far away that's when I start looking for the turn and I can usually spot the street sign even before I am close enough to read it. Yes in a neighborhood with streets close together it's a little more difficult, but as I keep looking for the street signs I can usually make the correct turn.

Of course sometimes no matter how hard you try you can't see the street sign and you don't know whether or not to turn on the next upcoming street. In those cases you have to rely on the GPS to tell you when to turn, and cross your fingers that it is correct. Well mentally cross your fingers since you need to keep both hands on the wheel

 

There is a delay; the GPS satellites are farther away than the local radio station.
The question I have is if the system is giving you enough notice of the turn's approach? The guidance software can compensate for the satellite communication delay.

 

GPS.gov: GPS Accuracy

 

I don't see how far away the GPS satellites are has any effect on the observed lag.With geostationary satellites, you see a lag when there is two-way communication going on, since you have 4 traversals to a satellite 23,000 miles away for each interaction. OTOH, the GPS satellites are 10,000 miles away and continuously transmitting their signal, and the GPS receiver does some complicated math on the signals received from several satellites to compute your location. How far away the satellite is won't induce any lag. GPS units are receive-only; there is no back-and-forth. The time the signal was transmitted is part of the continuously received data and part of the computation. Any observed lag would be from the time to do the computations and rendering on a map.

Given that radio signals travel at 186,000 miles/second, the fact that a consumer device can compute your location to within 16 feet based on the computed distance to several satellites orbiting thousands of miles away, and do it withing a second or two, is amazing.

 

That may account for perceived differences, some systems for example will show you continuing on the same route as you go through a tunnel for example even though the signal is lost, it assumes that you are still on the same road at the same speed. But eventually it will have to give up if it doesn't get a signal.

Not sure if it's related but my Garmin's sometimes can't figure out if I am on a frontage road or the freeway. It sometimes shows me doing some amazing aerial feats jumping back and forth! Although my newer Garmin's are much better, either because of better accuracy or predictive software, or both. And I'm not referring to when they turned off Selective Availability back in year 2000, prior to which the signal for civilians was intentionally degraded. When they turned that off I didn't notice much difference, probably because low-priced consumer units back then weren't as accurate anyway.